Heritability
heritability coefficient
Quantitative genetics produces a number called a heritability coefficient that is meant to provide a single measure of genetics’ influence on a trait (the number is between 0 to 1, with 1 being the strongest genetic influence). In a general way, a heritability coefficient measures how strongly differences among individuals are related to differences among their genes. For example, let’s say the heritability coefficient for height is 0.8. This means that 80% of the variation in height within a population can be explained by genetics, while the other 20% is due to environmental factors like nutrition and lifestyle.
In research compiled from studies of identical twins (in the Minnesota Study of Twins Reared Apart) researchers found that identical twins raised apart exhibit a higher correlation between their IQ scores than siblings or fraternal twins raised together (Bouchard, Lykken, McGue, Segal, & Tellegen, 1990).
Intelligence in the normal range is a polygenic trait, meaning that it is influenced by more than one gene, and in the case of intelligence at least 500 genes. Further, explaining the similarity in IQ of closely related persons requires careful study because environmental factors may be correlated with genetic factors. Twin studies of adult individuals have found a heritability of IQ between 57% and 73%, with some recent studies showing heritability for IQ as high as 80%. Some of the traits that the twin study also reported as having more than 50% heritability ratio include leadership, obedience to authority, a sense of well-being, alienation, resistance to stress, and fearfulness. It’s important to understand that heritability doesn’t mean that a trait is completely determined by genes. Instead, it tells us how much of the differences in a trait within a population can be explained by genetics. So, heritability helps us understand the role of genes in shaping our traits, but it’s not the whole story.
Genetics and Schizophrenia
The heritability of schizophrenia is said to be around 79%. This does NOT mean that a person with a parent with schizophrenia has a 79% chance of having schizophrenia, but rather that the 79% of the variation among people who either have or don’t have schizophrenia can be explained by genetics. Let’s examine this further and look at some findings on schizophrenia in light of what we know about gene-environment interactions.
In a 2004 study by Tienari and colleagues, of people who were given up for adoption, adoptees whose biological mothers had schizophrenia and who had been raised in a challenged family environment were much more likely to develop schizophrenia or another psychotic disorder than were any of the other groups in the study:
- Of adoptees whose biological mothers had schizophrenia (high genetic risk) and who were raised in challenged family environments, 36.8% were likely to develop schizophrenia.
- Of adoptees whose biological mothers had schizophrenia (high genetic risk) and who were raised in healthy family environments, 5.8% were likely to develop schizophrenia.
- Of adoptees with a low genetic risk (whose mothers did not have schizophrenia) and who were raised in challenged family environments, 5.3% were likely to develop schizophrenia.
- Of adoptees with a low genetic risk (whose mothers did not have schizophrenia) and who were raised in healthy family environments, 4.8% were likely to develop schizophrenia.
The study shows that adoptees with high genetic risk were most likely to develop schizophrenia if they were raised in challenged home environments. This research lends credibility to the notion that both genetic vulnerability and environmental stress are necessary for schizophrenia to develop, and that genes alone do not tell the full tale.
Final Thoughts on the Nature-Nurture Debate
So far, scientists agree that everything has some footing in genetics. But keep in mind, no behavioral traits are completely inherited, so you can’t leave the environment out altogether, either. The more genetically-related people are, the more similar they are—for everything: height, weight, intelligence, personality, mental illness, etc. Sure, it seems like common sense that some traits have a genetic bias. For example, identical twins are more similar to each other than fraternal twins. And while certain psychological traits, such as personality or mental illness (e.g., schizophrenia), seem reasonably influenced by genetics, it turns out that the same is true for political attitudes, how much television people watch (Plomin, Corley, DeFries, & Fulker, 1990), and whether or not they get divorced (McGue & Lykken, 1992).
The outcomes of nature–nurture studies have fallen short of our expectations of establishing clear-cut bases for traits in many ways. The most disappointing outcome has been the inability to organize traits from more- to less-genetic. Everything has turned out to be at least somewhat heritable (passed down through nature), yet nothing has turned out to be absolutely heritable, and there hasn’t been much consistency as to which traits are more heritable and which are less heritable once other considerations (such as how accurately the trait can be measured) are taken into account (Turkheimer, 2000). What we do know is that genes play an important role, but even they can change over time, and they still are only part of the full story.